Butyl polymers have been known and commercially available for many years. They possess a variety of inherently satisfactory properties as elastomers which has enabled them to find utility in many commercial areas. Among their satisfactory inherent properties are their impermeability to air, high damping of low frequency vibrations and good resistance to aging, heat, acids, bases, ozone and other chemicals, after vulcanization. These properties render butyl polymers well suited for use in a variety of applications including articles requiring low or reduced permeability to air. Non-limiting examples of such applications include tire inner tubes, tire curing bladders and various air bladders.
Halogenated butyl polymers have also been known and commercially available for many years. In addition to possessing the satisfactory inherent properties of butyl polymers described above, halogenated butyl polymers also possess cure compatibility with more highly unsaturated rubbers and good adhesion to such other rubbers after vulcanization, which renders them well suited for use in pneumatic tire inner liners.
Despite the advances made in the art to date, there is still room for improvement.
One of the properties of butyl rubber which is commercially important is resistance to cold flow. Specifically, users of butyl rubber typically will calendar the polymer into continuous very long sheets. In some cases, the polymer may be premixed with one or more of a filler (e.g., carbon black), a plasticizer, a tackifier, an extender oil and the like—such a composition may be characterized as a vulcanizable composition.
Once the calendar sheets are produced, it is quite common to reversibly fold or “wig-wag” the sheets on a pallet such that the height of the folded sheet is a number of feet. If the vulcanizable composition is susceptible to cold flow during shipping or storage, the weight of the folded polymer sheet causes the surface coating of the sheet (particularly at the bottom of the pile) to break, allowing adjacent fresh surfaces to contact, which leads to sticking of the sheets. This problem is exacerbated during long transportation or storage times, and/or high ambient temperatures. By the time the vulcanizable composition is ready for further processing it is more difficult to remove as a single sheet compared to the manner in which it was placed on the pallet.
While this can be alleviated, to some extent, by putting less polymer on a pallet, this increases the number of pallets required to transport or store a given amount of vulcanizable composition, thereby increasing costs.
U.S. Pat. No. 4,754,793 (Mohammed) teaches butyl elastomer compositions having reduced permeability to gases. Specifically, there is taught a rubber composition comprising a butyl polymer, an α-methylstyrene homopolymer, carbon black in a curing system, and optionally, a hydrocarbon extender oil. Mohammed fails to teach or suggest any effect on cold flow properties of the butyl polymer conferred by the addition of the α-methylstyrene homopolymer.
Further, to the knowledge of the inventors, α-methylstyrene homopolymer is not available in quantities commercially significant to be used as an additive in butyl polymer compositions. The principal reason may be attributed to a toughening of environment laws which had the effect of making it impractical to produce large quantities of the material for use as an additive as contemplated by Mohammed. Accordingly, while the teachings of Mohammed are useful, the commercial significance thereof is restricted somewhat by the unavailability of significant commercial quantities of α-methylstyrene homopolymer.
It would be desirable to have a butyl polymer composition having improved cold flow properties and a desirable balance of other properties, such as air impermeability, green strength and processability.
It is the object of the present invention to provide a novel butyl polymer composition having improved cold flow properties.
It is another objection of the present invention to provide a novel process for producing a butyl polymer vulcanizate.